Traffic lights are indispensable, especially at busy intersections. Unfortunately, the effectiveness of a traffic light is limited by its power source. During a blackout, traffic signals lose their utility line AC source of power and go dead. It is at this time, especially at night or when visibility is reduced, that intersections can become extremely perilous. Vehicles coming from other roads or directions are not adequately warned of approaching danger. In fact, many accidents occur every year during just such scenarios.
In the past, battery operated backup systems were not feasible because of the substantial power requirements for the incandescent light bulbs. Thus, a backup battery would quickly lose its charge unless, of course, unrealistically very large, expensive high capacity batteries were used. Furthermore, as traffic lights have flourished, they have become increasingly standardized in their size, control and operation.